The prospect of being denied engine service under warranty because of using a non-API oil or an oil with viscosity other than what is recommended is a constant theme here. I believe that it drives some decisions about oil choice that may not be optimal, based on driver habit, environment, vehicle usage, etc. Let's see how much of a real-world issue it is here, and how much is it mythology.
1. Have you ever been denied warranty service because you used a non-API oil?
2. Have you ever been denied warranty service because you deviated from the viscosity recommended/required in the manual?
3. Has a dealer ever questioned what viscosity or brand of oil you used for the purpose of determining your compliance with the manual?